Conventionally, as related art documents relating to a secondary air supply control apparatus of an internal combustion engine, JP-A-5-171973 is known. This document discloses a technique to warm up a catalyst early without using a high capacity air pump.
In the above related art, an air-fuel ratio supplied to the internal combustion engine is set to a rich side and secondary air is supplied, so that the catalyst can be early warmed up and activated. Conventionally, an air-fuel ratio feedback control during supply of secondary air has not been generally carried out since it is difficult. Thus, there has been a disadvantage that when the air-fuel ratio is disturbed by some disturbance factor during the supply of the secondary air, the engine rotation speed is much changed and the drivability becomes worse.
In order to cope with this, it is effective to execute the air-fuel ratio feedback control during the supply of the secondary air, however, when setting of a target air-fuel ratio at the time of start of the execution is not suitable, the air-fuel ratio supplied to the internal combustion engine is suddenly changed by this target air-fuel ratio, and a change occurs in the engine rotation speed, and as a result, there is fear that there occurs such a disadvantage that the drivability becomes worse.
Besides, according to JP-A-6-212959, after an upstream side catalyst is activated among plural catalysts, and when secondary air is being supplied to an exhaust system, heat energy, together with exhaust discharged to the exhaust system from the internal combustion engine, to be conveyed to the plural catalysts is increased, and since the heat energy, together with the exhaust, to be conveyed to the respective catalysts is not increased before the activation of the upstream side catalyst, degradation of emission is suppressed by that, and warm-up of the catalyst can be quickly performed.
Besides, according to JP-A-2001-263050, since exhaust gas from an internal combustion engine is heated up to the temperature at which it can burn in an exhaust passage at an upstream side of a catalyst, afterburning occurs, and the catalyst can be warmed up early by its burning heat, and unburned HC (hydrocarbon) exhausted from the internal combustion engine is burned by the afterburning.